A dramatic August surge has catapulted New York City Public Advocate Bill de Blasio into a surprising double-digit lead with less than two weeks to go before the Sept. 10 Democratic primary.

As late as mid-July, when the mayor’s race was dominated by the reemergence of Anthony Weiner, Mr. de Blasio languished in fourth place in most polls, a side story to the new campaign of the charismatic candidate, who energized what many saw as a lackluster race.

But as Carlos Danger and Sydney Leathers receded from the political spotlight, de Blasio, the most liberal voice, began to quietly pick up those voters abandoning the self-imploding congressman from Queens.

Now de Blasio has the backing of 36 percent of likely Democratic voters, up from 10 percent in mid-July, according to a Quinnipiac poll. De Blasio’s emergence has even put the loping 6-foot, 5-inch public advocate within reach of the 40 percent threshold needed to avoid an Oct. 1 runoff.

Long-time front-runner Christine Quinn, the City Council speaker and frequent ally of Mayor Michael Bloomberg, is languishing at 21 percent, a statistical dead heat with former Comptroller Bill Thompson, who narrowly lost to Mayor Michael Bloomberg in the 2009 general election. (Mr. Bloomberg cannot run for a fourth term because of term limits.)

Even in a runoff, however, the Quinnipiac poll shows de Blasio easily beating Ms. Quinn, 59 percent to 30 percent, and Mr. Thompson, 52 percent to 36 percent.

There are larger implications for de Blasio’s sudden surge. As the most left-leaning of the top three candidates, he could not only become the first Democrat to lead New York in 20 years, but also help reinvigorate American liberalism by providing liberals with a high-profile office.

“Absolutely, it will provide a prominent bully pulpit to somebody who articulates well the ideas of the progressive wing of the Democrats,” says Kenneth Sherrill, professor emeritus of political science at Hunter College in New York. “He was a skilled political operative before he ran for office himself, and I don’t think he is going to shy away from that platform, if he gets to it.”

De Blasio, the campaign manager for Hillary Rodham Clinton’s successful Senate run in 2000, has called for a tax increase on New Yorkers earning $500,000 or more. He would redistribute this wealth to fund universal pre-kindergarten services, as well as fully-funded after-school programs for all middle-schoolers. He also wants to shore up struggling city hospitals with increased state funding and provide financial support for immigrants trying to start or grow a small business.

And de Blasio hasn’t hedged on his calls for greater restrictions on New York's controversial stop-and-frisk policy, which allows cops to waylay anyone they think might be involved in serious criminal activity.

“What de Blasio has done with great success has been to energize the activist base of the Democratic party,” says Professor Sherrill. “He is saying things that won the hearts liberal Democrats.”

He has also benefited from Quinn and Thompson splitting the pro-Bloomberg vote. “Lots of polls show that about an equal number of Democrats like and dislike the mayor, [and] those who like the mayor are distributed among Thompson and Quinn pretty equally,” Sherrill adds. “But de Blasio has gotten a monopoly on the ones who dislike the mayor.”

Many Democrats express some approval of Bloomberg’s tenure as mayor, but 65 percent of likely Democratic voters say New York City needs to take a new direction, according to the Quinnipiac poll. Only 25 percent say they prefer to continue the direction of the Bloomberg years.

The much-sought-after New York Times endorsement went to Quinn, however, and the paper expressed skepticism that de Blasio could get any of his agenda passed.

“Mr. de Blasio has been the most forceful and eloquent of the Democrats in arguing that New York needs to reset its priorities in favor of the middle class, the struggling and the poor,” the Times endorsement said. “And yet, Mr. de Blasio’s most ambitious plans ... need support in the State Capitol, and look like legislative long shots. Once a Mayor de Blasio saw his boldest ideas smashed on the rocks of Albany, then what?”

De Blasio, were he to win the Democratic nomination, might also face a tougher general election than Quinn or Thompson, who are more centrist.

“Quinn and Thompson until recently have been running general election campaigns,” says Sherrill. “They didn’t want to say or do things now that will come back to haunt them in late October and November.”

“De Blasio is doing the exact opposite: he is running a primary campaign,” he continues. “And if he wins, some of things he’s saying and doing now may make the mayoral election more competitive come November – that is, I think the Republican candidate for mayor, especially if it’s [Joe] Lhota, he may have a better shot against de Blasio than against Quinn or Thompson, for this very reason.”

Mr. Lhota, a former deputy mayor under Rudolph Giuliani and the former head of the city’s transportation authority, leads in the race for the Republican nomination for mayor.

“Now the question is, in the remaining two weeks of a campaign, are the attacks that are coming, and have begun, are they going to weaken de Blasio’s support?" says Sherrill. "Is he going to lose those people, or has he already won their hearts to the point that the attacks won’t have any great impact?”